To investigate hierarchical models of galaxy formation and evolution it is important to have large samples of true satellites, extending to the faintest possible luminosities. We have used Hα narrow-band imaging to search for star-forming satellite galaxies around 143 field spiral galaxies comparable to the Milky Way and found 63 candidate satellites associated with 47 host galaxies. A subset of 12 candidate satellites and their 11 host galaxies were spectroscopically observed to test if the candidates were true satellites and bound to their host. This confirmed that Hα imaging is a reliable method for searching for faint satellites. Within the overall sample of 63 satellites only ~6 could be considered analogous with the LMC and ~14 with the SMC.

The NGVS is mapping the Virgo Cluster with a depth making possible to detect very low surface brightness (LSB) structures, such as faint dwarf galaxies. To extract these from just above the sky noise and make statistical studies of their properties, we use the software MARSIAA (MARkovian Software for Image Analysis in Astronomy). This segmentation software uses a Markovian approach to classify pixels and identify low-surface brightness structures.

We investigate the possibility that Hercules, a recently discovered Milky Way satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules’ highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body (Adén et al. 2009a). The application of simple analytical techniques (Jin & Lynden-Bell 2007) on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (−16−22+6 kms−1, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km s−1) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6−2+9 kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the Milky Way’s gravitational potential, inducing its transformation into a stellar stream.

Using VIMOS (VLT) we carried out a spectroscopic survey around seven late-type spiral galaxies and we discovered 55 satellites. After obtaining the radial velocities and spatial distribution of the satellites, we analyze their properties.

We have measured 3-D kinematics of stars in Kapteyn’s Selected Area (SA) 76 that were selected to be Anticenter Stream (ACS) members on the basis of their kinematics and CMD positions. Mean stream kinematics from the 31 identified ACS members produce an orbit inclined by ~30° to the well-defined spatial distribution of the stream. We have explored possible explanations for this, and suggest that our data in SA 76 are measuring the motion of a kinematically cold sub-stream among the ACS debris, which was likely a fragment of the same infalling structure that created the larger ACS system.

We first review the results of the tidal stirring model for the transformation of gas-rich dwarf irregulars into dwarf spheroidals, which turns rotationally supported stellar systems into pressure supported ones. We emphasize the importance of the combined effect of ram pressure stripping and heating from the cosmic ultraviolet background in removing the gas and converting the object into a gas poor system as dSphs. We discuss how the timing of infall of dwarfs into the primary halo determines the final mass-to-light ratio and star formation history. Secondly we review the results of recent cosmological simulations of the formation of gas-rich dwarfs. These simulations are finally capable to produce a realistic object with no bulge, an exponential profile and a slowly rising rotation curve. The result owes to the inclusion of an inhomogeneous ISM and a star formation scheme based on regions having the typical density of molecular cloud complexes. Supernovae-driven winds become more effective in such mode, driving low angular momentum baryons outside the virial radius at high redshift and turning the dark matter cusp into a core. Finally we show the first tidal stirring experiments adopting dwarfs formed in cosmological simulations as initial conditions. The latter are gas dominated and have turbulent thick gaseous and stellar disks that cannot develop strong bars, yet they are efficiently heated into spheroids by tidal shocks.

Dwarf galaxies (DGs) serve as extremely challenging objects in extragalactic astrophysics. Their origin is expected to be set as the first units in CDM cosmology. Nevertheless they are the galaxy type most sensitive to environmental influences and their division into multiple types with various properties have invoked the picture of their variant morphological transformations. Detailed observations reveal characteristics which allow to deduce the evolutionary paths and to witness how the environment has affected the evolution. Here we refer to general morphological DG types and review some general processes, most of which deplete gas-rich irregular DGs. Moreover, the variety of pecularities is briefly refered, but cannot be comprehensively analyzed because of limited paper space.

A widely held view is that a “mini halo” with much less than 108 M⊙ in dark matter is unlikely to retain any baryons because even a single SN event is expected to sweep out all of the gas. But we show that a clumpy medium is much less susceptible to SN sweeping (particularly if it is off-centred) because the coupling efficiency of the explosive energy is much lower than for a diffuse interstellar medium. With the aid of the sophisticated 3D hydro code Fyris, we show that baryons are retained and stars are formed in dark matter haloes down to 3 × 106 M⊙. The gas survives the SN explosion and is enriched with specific abundance yields of the discrete events. The smallest galaxies may not contribute a large fraction of baryons and dark matter to the formation of galaxies. But they are likely to carry important chemical signatures that were laid down in the earliest epochs of star formation, as we show. We discuss the results in light of the newly discovered, very metal poor, damped Lyα systems and ultra-faint dwarf galaxies.

We have investigated whether Smoothed Particle Hydrodynamics (SPH), equipped with artificial conductivity, is able to capture the physics of density/energy discontinuities in the case of the so-called shearing layers test, a test for examining Kelvin-Helmholtz (KH) instabilities. We can trace back each failure of SPH to show KH rolls to two causes: i) shock waves travelling in the simulation box and ii) particle clumping, or more generally, particle noise. The probable cause of shock waves is the Local Mixing Instability (LMI), previously identified in the literature. Particle noise on the other hand is a problem because it introduces a large error in the SPH momentum equation.

We show that setting up initial conditions with a suitably smoothed density gradient dramatically improves results. Particle clumping is easy to overcome, the most straightforward method being the use of a suitable smoothing kernel with non-zero first central derivative. We present results to that effect using a new smoothing kernel: the LInear Quartic (LIQ) kernel. Furthermore, we present new Artificial Conductivity signal velocities that lead to less diffusion.

The effects of the shock waves and of particle disorder become less important as the time-scale of the physical problem (for the shearing layers problem: lower density contrast and higher Mach numbers) decreases. At the resolution of current galaxy formation simulations mixing is probably not important. However, mixing could become crucial for next-generation simulations.

Dwarf irregulars (dIrr) and flat, rotating dwarf ellipticals (dE) generally posess flat metallicity profiles while round dEs show strong metallicity gradients (Koleva et al. 2009). Unlike dEs, dIrrs also exhibit ongoing star formation (SF) (Dolphin et al. 2005), in most cases compatible with a continuous star formation history (SFH). We show results based on a large suite of Nbody-SPH simulations of flat dwarf galaxies, both rotating and non-rotating, performed with a modified version of GADGET2. They have a range of masses, flattenings and rotations speeds and are based on the spherical models of (Valcke et al. 2008). Specifically, we want to see if it is possible to reproduce these characteristics in isolated DG models. These simulations show that using rotation to flatten a dwarf galaxy is particularly efficient in turning a so-called “breathing” SFH (Valcke et al. 2008) into a more continuous SFH, and in producing flat metallicity profiles. Non-rotating dEs in a flattened dark-matter halo are not able to reproduce these characteristics. Thus, it appears that rotation is key to reproducing the observed characteristics. Rotation causes a “centrifugal barrier” which slows down the infall of gas, so that the low-level star formation is not centrally concentrated but occurs galaxy-wide, and in this way also prevents large-scale oscillations in the SFR. This mechanism of smearing out the star formation in time and space proves to be the principal reason for the flat metallicity profiles, instead of the often referred to “fountain mechanism” (De Young & Heckman 1994; Barazza & Binggeli 2002; Mac Low & Ferrara 1999; Ferrara & Tolstoy 2000). We therefore propose a “centrifugal barrier mechanism” which is able to explain the observations.

We report recent results from the photometric follow-up study we are conducting in the context of the SAURON project. We use ground-based MDM V −band and Spitzer/IRAC 3.6 μm imaging to characterise our sample of E, S0 and Sa galaxies photometrically. Combined with SAURON integral-field spectroscopic observations, this information allows us to explore and understand the location of these galaxies on the Fundamental Plane relation, providing an important diagnostic tool to study their formation and evolution.

A chemical evolution of the Local Group dwarf galaxy Draco is considered. The stellar metallicity distribution function of Draco was calculated in the framework of both the monolithic and the merger scenario. The observed metallicity distribution isn’t reproduced quite well by monolithic collaps for this galaxy. The use of a merger of several fragments allows to obtain a better resemblance between modelled and observed stellar metallicity distributions.

Within the Cold Dark Matter scenario the hierarchical merging paradigm is the natural result to form massive galactic halos by the minor mergers of sub-halos and, by this, inherently their stellar halo. Although this must be also invoked for the Milky Way, the context of chemical and kinematic coherence of halo stars and dwarf spheroidal galaxies is yet unsolved a focus of present-day research. To examine this issue we model the chemo-dynamical evolution of the system of satellites selected from the cosmological Via Lactea II simulations to be similar for the Milky Way environment but at an early epoch.

We identify a high frequency of Type 1 XUV disks, reflecting recent outer-disk star formation, in a sample of 31 E/S0s with stellar masses primarily below M∗ ~ 4 × 1010 M⊙. Our ~40% identification rate is roughly twice the 20% fraction reported for late-type galaxies. Intriguingly, in the dwarf mass regime (below M∗ ~ 5 × 109 M⊙) where gas fractions increase, Type 1 XUV disks occur in ~70% of red-sequence E/S0s but only ~20% of blue-sequence E/S0s, a population recently linked to active disk rebuilding, especially in the dwarf regime. Our statistics are preliminary, but could indicate that Type 1 XUV disks in dwarf E/S0s are primarily related to weak/inefficient outer-disk star formation rather than to star formation capable of driving substantial disk growth.

The small (LV ≤ 107LV,⊙) baryonic components of the Milky Way’s dwarf spheroidal (dSph) satellites contribute small “backgrounds” with regards to estimates of dSph dark matter density profiles and any related predictions of dark matter decay/annihilation signals. As dSphs gain attention from a broad range of physicists interested in dark matter, it becomes important to examine the robustness – with respect to details like modeling assumptions and observational uncertainties – of conclusions regarding the dark matter content of dSphs and the scaling relations derived therefrom. Indeed dSphs seem to contain copious amounts of dark matter. However, the standard Jeans analysis constrains the dark mass enclosed only within the luminous scale radius, and does not reveal the internal dark matter distribution in a model-indepdendent way. Furthermore, new observational results help to identify complexities in internal dSph kinematics and cast doubt on previously apparent similarities among dSph dark matter halos. Further progress in understsanding dark matter on small scales will require consideration of dSphs more as individual galaxies and less as laboratories.

We apply a simple, one-equation, galaxy formation model on top of the halos and subhalos of a high-resolution dark matter cosmological simulation to study how dwarf galaxies acquire their mass and, for better mass resolution, on over 105 halo merger trees, to predict when they form their stars. With the first approach, we show that the large majority of galaxies within group- and cluster-mass halos have acquired the bulk of their stellar mass through gas accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are not built up by galaxy mergers. With the second approach, we constrain the star formation histories of dwarfs by requiring that star formation must occur within halos of a minimum circular velocity set by the evolution of the temperature of the IGM, starting before the epoch of reionization. We qualitatively reproduce the downsizing trend of greater ages at greater masses and predict an upsizing trend of greater ages as one proceeds to masses lower than mcrit. We find that the fraction of galaxies with very young stellar populations (more than half the mass formed within the last 1.5 Gyr) is a function of present-day mass in stars and cold gas, which peaks at 0.5% at mcrit = 106−8 M⊙, corresponding to blue compact dwarfs such as I Zw 18.

We use sub-parsec resolution hydrodynamic resimulations of a Milky Way (MW) like galaxy at high redshift to investigate the formation of the MW satellite galaxies. More specifically, we assess the impact of supernova feedback on the dwarf progenitors of these satellite, and the efficiency of a simple instantaneous reionisation scenario in suppressing star formation at the low-mass end of this dwarf distribution. Identifying galaxies in our high redshift simulation and tracking them to z = 0 using a dark matter halo merger tree, we compare our results to present-day observations and determine the epoch at which we deem satellite galaxy formation must be completed. We find that only the low-mass end of the population of luminous subhalos of the Milky-Way like galaxy is not complete before redshift 8.5, and that although supernovae feedback reduces the stellar mass of the low-mass subhalos (M ≤ 109Msun), the number of surviving satellites around the Milky-Way like galaxy at z = 0 is the same in the run with or without supernova feedback. If a luminous halo is able to avoid accretion by the Milky-Way progenitor before redshift 3, then it is likely to survive as a MW satellite to redshift 0.

We use the combination of the very large Millennium and Millennium-II simulations to follow galaxy formation over a stellar mass range of more than five orders of magnitude throughout representative cosmological volumes. These are the first simulations to address the abundance and spatial distribution of galaxies ranging from cD’s down to dSph’s. We demonstrate that stronger feedback is needed than in previous models, epically at low mass, if the SDSS stellar mass function is to be matched in a LCDM universe. Models which satisfy this constraint also reproduce the observed abundance of faint dwarfs around the Milky Way and M 31. Only the very faintest objects are significantly affected by the treatment of reionisation. Our model can reproduce most galaxy properties in the local Universe. Problems remain with respect to the stellar age distributions and kinematics of dwarfs.

Substructure in ΛCDM provides a number of interesting puzzles. While the missing satellites problem is well-studied, there are suggestions of an opposite problem on the bright end. Subhalos large enough to host luminous satellites are uncommon, so the existence of the Large Magellanic Cloud (LMC) orbiting the Galaxy can potentially be a challenge for ΛCDM. Hence, we describe a search for analogs to an isolated galaxy pair like the Milky Way/LMC system in the SDSS and interpret these results with cosmological simulations. We note that while the LMC may not be unusual based on its luminosity, it is remarkably blue for such satellites. Thus, color may have implications for the LMC’s orbital history.

High-resolution Hα velocity fields provide important observational constraints on the dark matter distribution in dwarf and low surface brightness galaxies. These two-dimensional data show that dark matter-dominated galaxies tend to be more consistent with cored halos than cuspy halos, at odds with theoretical expectations. Using N-body/SPH simulations of disk galaxy formation in cuspy spherical and triaxial dark matter halos, as well as cored dark matter halos, we “observe” the simulated galaxies under a variety of realistic observing conditions. We use these mock IFU velocity field observations to determine how well the underlying dark matter halo can be recovered and to test the hypothesis that cusps can be obscured by triaxial dark matter halos.